1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of Related Art
In general, liquid crystal displays are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an internal or external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light device. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFTs) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the backlight device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive LCD device has a transmittance of about 7.4% as seen in FIG. 1, which shows a transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device. However, this has a problem that the battery can not be used for a lengthy period of time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is light and easy to carry. Also, the reflective LCD device is superior in aperture ratio to the transmissive LCD device.
FIG. 2 shows a typical reflective LCD device in cross section. As shown in FIG. 2, the reflective LCD device includes upper and lower substrates 8 and 10 with a liquid crystal layer 12 interposed. The upper substrate 8 includes color filter layers 4a, 4b and 4c (e.g., red, green, and blue) and a common electrode 6. The lower substrate 10 includes a switching element (not shown) and a reflective electrode 2.
Ambient light 100 passes through the upper substrate 8 and the liquid crystal layer 12 and is reflected on the reflective electrode 2. When electrical signals are applied to the reflective electrode 2 by the switching element, phase of the liquid crystal layer 12 varies. Then, reflected light is colored by the color filter layers 4a, 4b and 4c and displayed in the form of images.
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that outdoors. Even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk).
In order to overcome the problems described above, a transflective LCD device has been developed. FIG. 3 shows a conventional transflective LCD device. As shown in FIG. 3, the conventional transflective LCD device includes upper and lower substrates 22 and 18 with a liquid crystal layer 20 interposed. The upper substrate 22 includes a color filter 104, and the lower substrate 18 includes a switching element (not shown), a pixel electrode 14 and a reflective electrode 2. The reflective electrode 2 is made of an opaque conductive material having a good reflectance and light transmitting holes xe2x80x9cAxe2x80x9d are formed therein. The transflective LCD device further includes a backlight device 16. The light transmitting holes xe2x80x9cAxe2x80x9d serve to transmit light 112 from the backlight device 16.
The transflective LCD device in FIG. 3 is operable in transmissive and reflective modes. First, in reflective mode, the incident light 110 from the upper substrate 22 is reflected on the reflective electrode 2 and directed toward the upper substrate 22. At this time, when electrical signals are applied to the reflective electrode 2 by the switching element (not shown), phase of the liquid crystal layer 20 varies and thus the reflected light is colored by the color filter 104 and displayed in the form of images.
Further, in transmissive mode, light 112 generated from the backlight device 16 passes through portions of the pixel electrode 14 corresponding to the transmitting holes xe2x80x9cAxe2x80x9d. When the electrical signals are applied to the pixel electrode 14 by the switching element (not shown), phase of the liquid crystal layer 20 varies. Thus, the light 112 passing through the liquid crystal layer 20 is colored by the color filter 104 and displayed in the form of images.
As described above, since the transflective LCD device has both transmissive and reflective modes, the transflective LCD device can be used without regard to the time of day (e.g., noon or dusk). It also has the advantage that it can be used for a long time by consuming low power. However, since the reflective electrode has the transmitting holes xe2x80x9cAxe2x80x9d, the conventional transflective LCD device has a very low light utilizing efficiency compared to either the reflective LCD device or the transmissive LCD device alone.
FIG. 4A shows a conventional reflective electrophoretic display. As shown in FIG. 4A, the reflective electrophoretic display 300 includes a front panel 24 having a first conducting electrode 25a and a first substrate 2a, and a rear panel 26 having a second conducting electrode 25b and a second substrate 2b. The display 300 also includes a suspension of charged pigment particles 30 colloidally dispersed in a dyed liquid 28 interposed between the front and rear panels 24 and 26.
The reflective electrophoretic display 300 operates as follows. As shown in FIG. 4B, when a d.c. voltage is applied to the first and second conducting electrodes 25a and 25b, charged pigment particles 30 contained in a dyed liquid 28 move and are packed on the first conducting electrode 25a having the same polarity so that the pigment particle layer 30a is formed. If polarity of the d.c. voltage is changed, the charged pigment particles 30 are packed (not shown) on the second conducting electrode 25b. When the pigment is packed on the first conducting electrode 25a, the color of the pigment will be seen by the observer with ambient light. When the pigment is packed on the second conducting electrode 25b, the ambient room light is absorbed and scattered by the dyed liquid 28 and the color of the dye is observed.
However, such a display utilizing electrophoresis has a problem that its response speed is slow, and it requires a high operating voltage. Further, pixelization is technically difficult. Also, the performance of the electrophoretic display above is far inferior to the LCD device.
Accordingly, the present invention is directed to a transflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
In accordance with the purpose of the invention, as embodied and broadly described, in one aspect the invention includes a transflective liquid crystal display device, including: a liquid crystal panel including a first substrate having a color filter, a second substrate having a switching element and a pixel electrode, and a liquid crystal layer interposed between the first and second substrates; an electrophoretic display arranged under the liquid crystal panel, the electrophoretic display including a first panel having a first conducting electrode which defines a transmitting region for transmitting light at a location corresponding to the pixel electrode, a second panel having a second conducting electrode, and a transparent liquid interposed between the first and second panels and having charged pigment particles; and a backlight device arranged under the electrophoretic display.
In another aspect, the invention includes a transflective liquid crystal display device, including: a liquid crystal panel including first and second substrates with liquid crystal material therebetween arranged to define a plurality of pixels, the second substrate including a switching element and a transparent electrode in each pixel; an electrophoretic display arranged under the liquid crystal panel, the electrophoretic display including a first panel having a first conducting electrode which defines a plurality of transmitting regions for transmitting light in each pixel at locations corresponding to the transparent electrodes, a second panel having a second conducting electrode, and a transparent liquid interposed between the first and second panels and having charged particles therein, wherein a voltage applied between the first and second conducting electrodes determines whether the transflective liquid crystal display device operates in a transmissive mode or in a reflective mode; and a backlight device arranged under the electrophoretic display.
In still another aspect, the invention includes a method of operating a transflective liquid crystal display, the transflective liquid crystal display having a liquid crystal panel, a backlight, and an electrophoretic display arranged therebetween, the electrophoretic display including two electrodes and charged particles suspended in a liquid, the method including: applying a voltage having a polarity between the two electrodes to attract the charged particles to one of the two electrodes and allow light from the backlight to exit the transflective liquid crystal display; and applying a voltage having an opposite polarity between the two electrodes to attract the charged particles to another of the two electrodes to form a layer of particles which reflects light from external to the transflective liquid crystal display.
In another aspect, the invention includes a transflective liquid crystal display device, including: a liquid crystal panel having a plurality of switchable pixels operable to selectively transmit light; a backlight arranged under the liquid crystal panel; and an electrophoretic device arranged between the liquid crystal panel and the backlight operable to selectively transmit light from the backlight or reflect light from external to the transflective liquid crystal display device based on a polarity of a voltage applied thereto.
Preferred embodiments of the present invention provide a transflective liquid crystal display device advantageously having improved light utilizing efficiency, good contrast, and brightness.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.